Heretofore, fabrication of a MEMS inkjet print head presented difficulties by virtue of the very components being joined. In particular, the MEMS inkjet print head incorporates a MEMS membrane device and a driver substrate, each formed with processes that can be detrimental to the other.
Traditional MEMS membrane devices can be fabricated using thin film surface micromachining techniques. For example, polysilicon layers are deposited over sacrificial silicon glass layers and the sacrificial layers are dissolved through a multitude of etch holes to allow the etchant to flow underneath the membranes. This etch process can affect required passivation of microelectronic components and the required holes need to be hermetically sealed after the etch release in some cases to prevent the device from malfunctioning. The aggressive chemical etch is typically performed with hydrofluoric acid (HF), which limits material choices for the designer. Further, use of the chemical etch complicates an integration of MEMS devices with traditional microelectronic components such as a substrate driver used in the MEMS inkjet print head. In addition, released devices can be difficult to process with traditional microelectronic techniques creating yield loss or restricted design options.
Conventional circuit driver substrates designed as CMOS devices are commonly employed to drive transducers and reduce input/output lines. These can be complex assemblies of thin films passivated with silicon oxides. If this type of device is exposed to a strong etchant, such as HF, it might no longer function. While steps can be taken to protect these passivation layers, other MEMS processes, particularly high temperature processes such as polysilicon deposition and annealing, can adversely impact the operation of transistor circuits. This is also aggravated by compound yield effects of additional microelectronic layers. Accordingly, CMOS and MEMS present a challenge to integrate.
FIGS. 4A and 4B depict some basic features of a known MEMS inkjet print head and are provided to illustrate differences between the known heads and that of the exemplary embodiments.
In the known polysilicon membrane design of a MEMS inkjet print head, a larger, more complex structure 410 is used between adjacent membranes 420. These structures are used for sealing hydrofluoric acid etch release holes 430 in the membrane and for tolerance adjustments between membranes. In the exemplary embodiments described herein, a thinner, less complex fluid wall can be formed, and there are no holes in the membrane structure.
In order to form a print head device, the free membranes must be very small and at a very high density. For 600 nozzles per inch, the print head must have a pitch of 42.25 μm. This does not leave much room for sealing and alignment of the layers between each ejector nozzle.
Thus, there is a need to overcome these and other problems of the prior art and to provide a method and apparatus for a MEMS electrostatic inkjet print head in which the electrostatic membrane and drive electrode are fabricated on separate wafers prior to bonding the wafers together in an inkjet print head.